Wireless sensor networks (WSN) are deployed in many industrial and commercial environments. Applications in industrial environments offer an enormous business potential for these types of networks. A machine failure can cause large expenses if not detected, when compared to the cost of a systematic shut down of a machine for service well before the machine fails.
However, scalability, reliability, and latency are challenging issues. Wired sensor networks are generally used for such applications due to their high reliability and low latency. Such networks, however, are costly, complex, and inflexible. A change in topology of the network can mean reinstallation of the wired backbone. That can not be cost effective, and can force a prolonged down time.
This leads to a need for a reconfigurable, low cost, and less installation intensive sensor networking technology that satisfies the requirements of industrial applications. A viable solution is to use wireless sensor networks. These networks are both cheap to install and flexible to topological changes due to their relatively simple and fast setup procedures. The challenge, however, lies in providing a similar degree of reliability and latency as offered by their wired networks. Limited resources that are generally available to sensor nodes effectively preclude the use of forward error correction codes or other computationally intensive approaches.
Wireless data networks have been used in diverse environments. Wireless cellular networks, WiFi, Bluetooth, WiMax, and RF, for example, are all well suited to their respective application domains. A prime candidate technology for applications in industrial control and automation is the specification defined by IEEE 802.15.4 standard for the MAC and physical layers in a multi-hop wireless mesh network. However, the current IEEE 802.15.4-2006 standard fails to satisfy the stringent requirements for latency and reliability performance necessary for industrial deployments.
A number of MAC types are known, including carrier sense multiple accesses (CSMA), time division multiple access (TDMA), code division multiple access (CDMA), and frequency hopping (FH). Hybrid MAC types use a combination of CSMA, TDMA and FH.
The IEEE 802.15.4 standard specifies a physical layer and medium access control (MAC) for lower layers o a low-rate wireless personal area networks (LR-WPAN's). This standard is the basis for the ZigBee and MiWi specification, which offers networking solution for the upper layers, which are not covered by the standard.
The IEEE 802.15.4 standard offers fundamental lower network layers of a type of wireless personal area network (WPAN), which focuses on low-cost, low-speed ubiquitous communication between devices, in contrast with other, more end user-oriented approaches, such as Wi-Fi. The emphasis is on very low cost communications of nearby devices with little to no underlying infrastructure, intending to exploit this to lower power consumption even more.
Three approaches for accessing communication channels in wireless sensor networks have been commonly used. The first approach, called TDMA, partitions the time axis into slots, and then allocates these time slots to participating nodes for contention-free channel access. That approach is very useful in single-hop networks, such as wireless cellular phone systems, where network resources, such as time slots, channels, and communication signal power, etc., are managed by a central entity.
The central management approach is generally not very feasible and scalable for large multi-hop wireless sensor networks, where the channel condition can change frequently, routes can not be reliable, and the network topology can not be available. In such networks, managing network resources efficiently and keeping a central repository of resources' allocation updated is not a simple or feasible task.
More important, TDMA based systems lack the required flexibility to handle failed transmissions, which are common in wireless sensor networks, because of their inability to provide opportunities for retransmission. Moreover, because the time slots are generally allocated in a static non-adaptive manner, those systems are normally ill prepared to handle burst traffic. Some approaches delegate the responsibility of resource allocation in such networks to higher layers. That can result in two undesirable factors. First, the network becomes inefficient. The latency in responding to changes in a volatile operating environment, which wireless sensor networks normally operate in, makes the network less adaptive. Secondly, it makes the design of higher layers, including the application network designs, more difficult because of limited familiarity with the issues and problem related to the lower level network components.
A second approach, called CSMA, allows every node to try to access the channel whenever node needs to transmit a frame. The node, however, “listens” on the channel before starting transmission to ensure that the transmission will not interfere with an already transmitting node. This approach needs minimum degree of resource allocation. However, throughput is degraded due to collisions between transmissions.
A third approach for channel access and network resource management is to use a hybrid network that allows both the contention-based CSMA channel access as well as contention-free TDMA channel access by the use of a well-defined structure, called a MAC super frame. The traffic in the TDMA part is due to the frames transmitted by the nodes in their allocated time slots. The network traffic in the CSMA part is meant to satisfy the need for asynchronous communications, which are generated by management frames, requests for time-slot allocation, transmission of normal data frames, and retransmissions of failed TDMA data frames.
The channel access times are specified in the form of a MAC super frame and periodically announced in a beacon. Each full functional device (FFD) generally “owns” a super frame. The IEEE 802.15.4-2006 standard follows this hybrid approach.
FIG. 1 shows a super frame 160 according to the EEE 802.15.4 standard. The horizontal axis 105 indicates time. Each coordinator in the network periodically transmits a beacon 100. The beacon is used for synchronization and resource allocation. An interval between two consecutive beacons is a beacon interval 120.
The super frame includes a contention access period (CAP) 150 that uses CSMA, followed by a contention free period (CFP) that uses TDMA. The CFP 140 includes guaranteed time slots (GTS) 145. Each time slots 145 is allocated to a device that requires contention free access to the channel to minimize probability of collision of its transmission with other transmissions. Typically, the CFP is used for more important traffic that must get though in time.
The CAP 150 and the CFP 140 form the active portion 110 of the super frame 160, which is followed by a much longer inactive period 130. The inactive period can be used by other coordinators, while the coordinator device of this super frame is idle and ‘listens’ to the channel for transmissions by the other coordinators. A child coordinator 11 can start its super frame 170 during the inactive portion 130 of the super frame 160 of its parent coordinator 12. A leaf node communicates with its parent coordinator only during the active portion 110 of the super frame 160 of its parent coordinator 10. The inactive period can be several seconds.
However, the IEEE802.15.4-2006 standard fails to satisfy the performance requirements for industrial deployments, including scalability, reliability, and latency issues.
Moreover, it is desirable that frequency channel hopping should be used for a higher reliability and better channel efficiency. It is also desirable to allocate channels dynamically for retransmission of failed transmissions and additional channel access should be provided dynamically on demand in the same super frame in order to better handle sudden increase in data traffic.